Power-supply unit

ABSTRACT

A power-supply unit includes a high-voltage source that generates a high voltage between a positive electrode and a negative electrode, a smoothing capacitor connected between the positive electrode and the negative electrode, a discharge portion that includes a resistor and a first switching device connected in series with each other, and is connected between the positive electrode and the negative electrode, and a discharge control portion that controls the first switching device to one of an ON state and an OFF state. When an abnormal condition in which current flows through the resistor is detected while the discharge portion controls the first switching device so as to keep the first switching device in the OFF state, the high-voltage source is controlled so as to keep generating a given high voltage for fusing the resistor.

INCORPORATION BY REFERENCE

The disclosure of Japanese Patent Application No. 2013-034630 filed onFeb. 25, 2013 including the specification, drawings and abstract isincorporated herein by reference in its entirety.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The invention relates to a power-supply unit for supplying electricpower to a load unit, such as a motor for driving a vehicle, and a motorfor driving a machine, for example.

2. Description of Related Art

Generally, a power-supply unit for supplying electric power to a loadunit, such as a driving motor, often includes a switching circuit, suchas an inverter. Accordingly, the power-supply unit of this type oftenincludes a smoothing capacitor. Since the smoothing capacitor isconnected between a positive electrode and a negative electrode of thepower-supply unit, the voltage between the opposite ends of thecapacitor is relatively high. Thus, a large quantity of electric chargeis stored in the smoothing capacitor. Therefore, when an abnormalityoccurs to the vehicle, machine, or the like, it is desired to quicklydischarge the smoothing capacitor (quickly release the electric chargestored in the smoothing capacitor).

In one example of the related art concerning the power-supply unit forthe vehicle, a discharge portion (rapid discharge circuit) including aresistor (resistive element) and a switching device (transistor) isarranged in parallel with the smoothing capacitor, and the switchingdevice is switched to an ON state when a collision of the vehicle isdetected. As a result, the smoothing capacitor is rapidly dischargedafter detection of the vehicle collision (see, for example, JapanesePatent Application Publication No. 2012-186887 (JP 2012-186887 A).

However, if the switching device of the discharge portion is broughtinto a condition of a short-circuit fault, for example, the voltagebetween the positive electrode and the negative electrode is reduced;therefore, sufficient electric power cannot be supplied to the loadunit. Accordingly, where the load unit is a motor for driving a vehicleor a machine, the vehicle or machine may not continue to be normallyoperated.

SUMMARY OF THE INVENTION

The invention provides a power-supply unit that is able to continue tosupply electric power to a load unit, by cutting off a discharge currentpathway formed by a discharge portion, when an abnormal condition inwhich a smoothing capacitor is discharged via the discharge portion in asituation where the smoothing capacitor should not be discharged isdetected.

A aspect of the invention is concerned with a power-supply unitincluding a high-voltage source configured to generate a high voltagebetween a positive electrode and a negative electrode so as to supplyelectric power to a load unit connected to the positive electrode andthe negative electrode, a smoothing capacitor connected to the positiveelectrode and the negative electrode, a discharge portion that includesa resistor and a first switching device connected in series with eachother, and is connected to the positive electrode and the negativeelectrode, a discharge control portion configured to control the firstswitching device to one of an ON state and an OFF state, and anabnormality detecting portion configured to detect occurrence of anabnormal condition in which electric current flows through the resistoreven though the discharge control portion controls the first switchingdevice so as to keep the first switching device in the OFF state. Thesmoothing capacitor and the discharge portion are configured such thatan electric charge of the smoothing capacitor is discharged by thedischarge portion when the first switching device is in the ON state.

Furthermore, the power-supply unit according to the aspect of theinvention includes a forcedly cutting-off portion configured to forcedlycut off a discharge current pathway formed by the discharge portion,when the abnormal condition is detected.

With the above arrangement, when the above-described abnormal conditionis detected, the forcedly cutting-off portion forcedly cuts off thedischarge current pathway; therefore, the voltage between the terminalsof the smoothing capacitor is not reduced, and electric power can bekept supplied to the load unit.

Accordingly, when the power-supply unit is used as a device forsupplying electric power to a motor for driving a vehicle as a loadunit, it is possible to keep the vehicle running.

When the abnormal condition is detected, the forcedly cutting-offportion may be configured to control the high-voltage source so that thehigh-voltage source keeps generating a given high voltage for fusing theresistor of the discharge portion.

With the above arrangement, when the above-described abnormal conditionis detected, the resistor of the discharge portion fuses due to heatgenerated by the resistor, so that the discharge current pathway is cutoff; therefore, the voltage between the terminals of the smoothingcapacitor is not reduced, and electric power can continue to be suppliedto the load unit.

The discharge portion may include a second switching device connected inseries with the resistor and the first switching device, and theforcedly cutting-off portion may be configured to switch the secondswitching device from an ON state to an OFF state when the abnormalcondition is detected.

With the above arrangement, when the above-described abnormal conditionis detected, the second switching device is placed in the OFF state;therefore, the voltage between the terminals of the smoothing capacitoris not reduced, and electric power can continue to be supplied to theload unit.

BRIEF DESCRIPTION OF THE DRAWINGS

Features, advantages, and technical and industrial significance ofexemplary embodiments of the invention will be described below withreference to the accompanying drawings, in which like numerals denotelike elements, and wherein:

FIG. 1 is a schematic view showing the configuration of a power-supplyunit, load unit, and a drive unit of a vehicle according to a firstembodiment of the invention;

FIG. 2 is a flowchart illustrating a routine executed when a CPU of anintegrated control device shown in FIG I performs a forcedly cutting-offoperation;

FIG. 3 is a view useful for explaining a method of designing a dischargeresistor shown in FIG. 1;

FIG. 4 is a schematic view showing the configuration of a power-supplyunit, load unit, and a drive unit of a vehicle according to a secondembodiment of the invention; and

FIG. 5 is a flowchart illustrating a routine executed when a CPU of anintegrated control device shown in FIG. 4 performs a forcedlycutting-off operation.

DETAILED DESCRIPTION OF EMBODIMENTS

A power-supply unit according to each embodiment of the invention willbe described with reference to the drawings. The power-supply unit ofeach embodiment is applied to a hybrid vehicle. It is, however, to beunderstood that the invention may also be applied to vehicles, such asan electric vehicle and a fuel-cell vehicle, and systems, such asmachine tools, ships and aircraft, including a load unit (e.g., a motor)using electric power supplied from a high-voltage power supply.

First Embodiment

(Configuration) As shown in FIG. 1, a power-supply unit (which will alsobe called “first power-supply unit”) 11 according to a first embodimentof the invention is installed on a hybrid vehicle (which will also becalled “vehicle”) 10. Further, a load unit 12 and a drive unit 13 areinstalled on the vehicle 10.

The power-supply unit 11 includes a high-voltage source HVS, a smoothingcapacitor portion SC, and a discharge portion DCHG.

The high-voltage source HVS includes a storage battery 20, a boostconverter 30, and system main relays SMR1-SMR3.

The storage battery 20 is a chargeable/dischargeable secondary battery,which is a lithium-ion battery in this embodiment. The storage battery20 generates DC power to a pair of storage-battery terminals P1, N1. Thestorage battery 20 is charged with voltage applied from the outside tothe pair of storage-battery terminals P1, N1.

The boost converter 30 has a pair of low-voltage-side terminals P2, N2,and a pair of high-voltage-side terminals P3, N3. The boost converter 30includes a capacitor 31, reactor 32, first transistor (power MOSFET) 33,diode 34, second transistor (power MOSFET) 35, and a diode 36. Theseelements constitute a known boost chopper circuit as shown in FIG. 1.

By using the boost chopper circuit, the boost converter 30 can convert“a low-voltage-side voltage VL substantially equal to a voltage (i.e.,storage-battery voltage) between the pair of storage-battery terminalsP1, N1” into “a high-voltage-side voltage VH as a voltage between thepair of high-voltage-side terminals P3, N3)”, and vice versa. Namely,the first transistor 33 and the second transistor 35 are switched basedon a PWM (Pulse Width Modulation) signal from an integrated controldevice 100 (which will be described later), so that the boost converter30 can perform a boosting or step-up operation to convert thelow-voltage-side voltage VL to the high-voltage-side voltage VH, and astep-down operation to convert the high-voltage-side voltage VH to thelow-voltage-side voltage VL. The operation of the boost converter 30 iswell known, and therefore will not be further described.

The system main relays (which will be called “relays) SMR1-SMR3 aredevices that operate in conjunction with “a power switch of the vehicle10” (not shown) to connect and disconnect the storage battery 20 to andfrom the boost converter 30. The relay SMR1 is connected between theterminal N1 and one end of a resistor RL. The other end of the resistorRL is connected to the terminal N2. The relay SMR2 is connected betweenthe terminal Ni and the terminal N2. The relay SMR3 is connected betweenthe terminal P1 and the terminal P2. The relays SMR1-SMR3 are opened andclosed according to a signal from the integrated control device 100.

The smoothing capacitor portion SC includes a smoothing capacitor 40.The smoothing capacitor 40 is connected between the terminal P3 and theterminal N3, and smoothens ripples generated between the terminal P3 andthe terminal N3.

The discharge portion DCHG includes a rapid discharge circuit 50. Therapid discharge circuit 50 is connected in parallel with the smoothingcapacitor 40. Namely, the rapid discharge circuit 50 is connectedbetween the terminal P3 and the terminal N3. The rapid discharge circuit50 includes a discharge resistor 51, switching device 52, and adischarge current sensor 53. The discharge resistor 51, switching device52, and the discharge current sensor 53 are connected in series. In thisembodiment, the discharge current sensor 53 is a shunt resistor. Theswitching device 52 is also called “first switching device” for the sakeof convenience. The switching device 52 is a power MOSFET.

The load unit 12 includes a first inverter 60, second inverter 70, firstmotor 81, and a second motor 82.

The first inverter 60 has a pair of input terminals P4, N4. The pair ofinput terminals P4, N4 are respectively connected to the pair ofhigh-voltage-side terminals P3, N3 of the boost converter 30. The firstinverter 60 includes a U-phase arm, V-phase arm, and a W-phase arm. Eachof these arms is inserted between the pair of input terminals P4, N4,and these arms are connected in parallel with each other.

The U-phase arm of the first inverter 60 has an IGBT 61 s and an IGBT 62s. A diode 61 d and a diode 62 d are connected in inverse parallel withthe IGBT 61 s and the IGBT 62 s, respectively. The IGBT 61 s and theIGBT 62 s are connected in series with each other. A point of connectionbetween the IGBT 61 s and the IGBT 62 s is connected to a U-phase coil(not shown) of the first motor 81.

The V-phase arm of the first inverter 60 has an IGBT 63 s, diode 63 d,IGBT 64 s, and a diode 64 d. The relationship of connection among theseelements is identical with that of the U-phase arm, as shown in FIG. 1,and a point of connection between the IGBT 63 s and the IGBT 64 s isconnected to a V-phase coil (not shown) of the first motor 81.

The W-phase arm of the first inverter 60 has an IGBT 65 s, diode 65 d,IGBT 66 s, and a diode 661 The relationship of connection among theseelements is identical with that of the U-phase arm, as shown in FIG. 1,and a point of connection between the IGBT 65 s and the IGBT 66 s isconnected to a W-phase coil (not shown) of the first motor 81.

By using these devices, the first inverter 60 converts DC power receivedfrom the boost converter 30 into three-phase AC power of the U phase, Vphase and W . phase, and delivers the AC power to the first motor 81,according to a signal from the integrated control device 100. Theoperation of the first inverter 60 is well known, and therefore, willnot be further described.

The second inverter 70 is configured similarly to the first inverter 60.Namely, a pair of input terminals P5, N5 of the second inverter 70 areconnected to the pair of high-voltage-side terminals P3, N3 of the boostconverter 30, respectively. The second inverter 70 includes IGBTs 71s-76 s and diodes 71 d-76 d. By using these devices, the second inverter70 converts DC power received from the boost converter 30 intothree-phase AC power of the U phase, V phase and W phase, and deliversthe AC power to the second motor 82, according to a signal from theintegrated control device 100. The operation of the second inverter 70is well known, and therefore, will not be further described.

The first motor 81 and the second motor 82 are synchronousgenerator-motors. Namely, each of the first motor 81 and the secondmotor 82 may operate as an electric motor and also operate as agenerator. The first motor 81 is mainly used as a generator. The secondmotor 82 is mainly used as an electric motor, and generates drivingforce of the vehicle 10 (torque for running the vehicle 10).

The drive unit 13 includes an internal combustion engine 83, power splitdevice 90, speed reducing device 91, drive shaft 92, differential gear93, and drive wheels 94.

The internal combustion engine 83 is a gasoline engine, and is able togenerate driving force of the vehicle 10. The intake air amount, fuelinjection amount, etc. of the internal combustion engine 83 arecontrolled based on signals from the integrated control device 100.

The power split device 90 includes a planetary gear mechanism, and isarranged to convert torque from the internal combustion engine 83, firstmotor 81 and second motor 82, and deliver the torque to the differentialgear 93 via the speed reducing device 91 and the drive shaft 92. Thetorque delivered to the differential gear 93 is transmitted to the drivewheels 94. The power split device 90 and its control method are wellknown, and are described in detail .in, for example, Japanese PatentApplication Publication No. 2009-126450 (JP 2009-126450 A) (U.S. PatentApplication Publication No. 2010/0241297), and Japanese PatentApplication Publication No. 9-308012 (JP 9-308012 A) (U.S. Pat. No.6,131,680 having a U.S. filing date of Mar. 10, 1997). Thesepublications are referred to herein, and thus incorporated into thespecification of this application.

The vehicle 10 further includes a control unit CNT. The control unit CNTincludes an integrated control device 100, collision detecting portion110, rapid discharge control circuit 120, and an abnormality detectingportion 130.

The integrated control device 100 includes a plurality of electroniccontrol units (ECUs) for controlling the vehicle 10. Namely, theintegrated control device 100 includes a power management ECU thatperforms integrated control of the driving force of the vehicle 10,battery charge, and so forth, MG-ECU that controls the first motor 81and the second motor 82, engine-ECU that controls the internalcombustion engine 83, battery-ECU that monitors the storage battery 20,and so forth. Each of the electronic control units is a microcomputerthat includes a CPU, memory, etc., and executes corresponding programs.The electronic control units exchange information with each other viacommunication lines.

The integrated control device 100 is connected to the storage battery20, relays SMR1-SMR3, boost converter 30, first inverter 60, secondinverter 70, collision detecting 110, rapid discharge control circuit120, and the abnormality detecting portion 130. The integrated controldevice 100 is configured to send a “discharge command signal” to therapid discharge control circuit 120, when it receives a collisiondetection signal from the collision detecting portion 110. Further, theintegrated control device 100 is configured to send a “resistor fusinghigh-voltage generation command signal” to the boost converter 30, basedon a signal from the abnormality detecting portion 130, when ashort-circuit fault as described later occurs.

The collision detecting portion 110 determines whether a collision ofthe vehicle 10 has occurred by a well-known method, based on a signalfrom a G sensor (acceleration sensor) installed at an appropriatelocation in the vehicle 10. When it is determined that a collision ofthe vehicle 10 has occurred, the collision detecting portion 110 sends acollision detection signal to the integrated control device 100.

When the rapid discharge control circuit 120 receives the dischargecommand signal from the integrated control device 100, it switches theswitching device 52 from a cut-off state (OFF) to an energized state(ON), so as to discharge the smoothing capacitor 40,

The abnormality detecting portion 130 receives a voltage across theopposite ends of the discharge current sensor 53. Since the dischargecurrent sensor 53 is a shunt resistor, the voltage across its oppositeends is proportional to current that flows through “a discharge currentpathway consisting of the discharge resistor 51 and the switching device52”. The abnormality detecting portion 130 compares the voltage receivedfrom the discharge current sensor 53 with a threshold value used fordetermining a short-circuit fault (abnormal condition), and sends theresult of comparison to the integrated control device 100.

The discharge resistor 51 is provided for discharging an electric chargestored in the smoothing capacitor 40 and reducing the voltage of thesmoothing capacitor 40 to a given voltage or lower (e.g., 60V or lower)within a given period of time (5 sec. or shorter), when a collision ofthe vehicle 10 is detected by the collision detecting portion 110. Onthe other hand, during normal running of the vehicle 10, the rapiddischarge control circuit 120 controls the switching device 52 so thatthe switching device 52 is kept in the “OFF” state.

When an abnormal condition (an abnormal condition where current flowsthrough the discharge resistor 51 and the switching device 52) in whichthe switching device 52 is placed in the “ON” state for some reason isdetected, even though the switching device 52 is controlled by the rapiddischarge control circuit 120 so as to be placed in the “OFF” state, theboost converter 30 is controlled so that the voltage across the pair ofhigh-voltage-side terminals P3, N3 is raised to a forcedly boostedvoltage, based on the above-mentioned “resistor fusing high-voltagegeneration command signal”. As a result, large current is caused to flowthrough the discharge resistor 51, and the discharge resistor 51 isdesigned to be fused or melted down due to the current flowingtherethrough. A method of designing the discharge resistor 51 of thistype will be described later.

Further, the vehicle 10 includes a voltmeter 21 and a voltmeter 22. Thevoltmeter 21 measures the low-voltage-side voltage VL, and sends it tothe integrated control device 100. The voltmeter 22 measures thehigh-voltage-side voltage VH, and sends it to the integrated controldevice 100.

The integrated control device 100 determines a target value of thehigh-voltage-side voltage VH based on the torque required of the vehicle10, and controls the boost converter 30 so that the actualhigh-voltage-side voltage VH detected by the voltmeter 22 coincides withthe target value. During running (normal running) of the vehicle 10, thetarget value of the high-voltage-side voltage VH is kept at a voltage(e.g., 200-400V) that is lower than the forcedly boosted voltage (e.g.,600V) as will be described later. However, the target value of thehigh-voltage-side voltage VH during normal running may be momentarilyset to a voltage equivalent to the forcedly boosted voltage.

(Operation) Next, the operation of the first power-supply unit 11constructed as described above will be described with regard to the caseof a collision of the vehicle 10, and the case of a short-circuit fault,respectively.

<Case of Collision> As described above, when the vehicle 10 comes intocollision, a collision detection signal is transmitted from thecollision detecting portion 110 to the integrated control device 100. Inresponse to the signal, the integrated control device 100 sends a“discharge command signal” to the rapid discharge control circuit 120.The rapid discharge control circuit 120, which has received this signal,performs control so as to bring the first switching device 52 of therapid discharge circuit 50 into the ON state. Accordingly, electriccurrent flows through the discharge resistor 51 of the rapid dischargecircuit 50, and an electric charge stored in the smoothing capacitor 40is discharged.

At the same time, the integrated control device 100 sends an “opencommand signal” to the relays SMR1-SMR3, so as to immediately stop theoperation of a high-voltage system of the power-supply unit 11. As aresult, the relays SMR1-SMR3 are immediately opened, and supply ofelectric power via the boost converter 30 is stopped. Accordingly, inthe event of the collision of the vehicle 10, the electric charge storedin the smoothing capacitor 40 is rapidly discharged.

<Case of Short-circuit Fault> As described above, when a short-circuitfault (abnormal condition, abnormal discharge condition) takes place,the integrated control device 100 sends a “resistor fusing high-voltagegeneration command signal” to the boost converter 30, based on a signalfrom the abnormality detecting portion 130. This point will be describedin more detail with reference to the flowchart of FIG. 2.

The CPU of the integrated control device 100 is configured to execute aroutine as illustrated in the flowchart of FIG. 2 each time a givenlength of time elapses. Thus, at an appropriate time, the CPU starts theroutine from step S200, and proceeds to step S210 to determine whetherthe CPU sends an “OFF” command to the first switching device 52 of therapid discharge circuit 50. In other words, the CPU determines whether“no discharge command signal is generated” at this point in time.

If the vehicle 10 is in a collision as described above, the CPU sends acommand signal for placing the first switching device 52 in the ON stateto the rapid discharge control circuit 120. Namely, the CPU generates adischarge command signal. In this case, the CPU makes a negative (“NO”)decision in step S210, and directly proceeds to step S295 to once finishthe routine.

If the vehicle 10 is not in a collision but in a normal runningcondition, the CPU sends a signal for controlling the first switchingdevice 52 to the OFF state, to the rapid discharge control circuit 120.In this case, the CPU makes an affirmative decision (“YES”) in stepS210, and proceeds to step S220 to determine whether the result ofcomparison transmitted from the abnormality detecting portion 130indicates “occurrence of a short-circuit fault (abnormal condition)”.

The “short-circuit fault (abnormal condition)” may occur for somereasons. For example, two reasons as follows may be considered.

(1) The interior of the first switching device 52 is in a constantlyshort-circuited condition due to insulation breakdown of the firstswitching device 52.

(2) The rapid discharge control circuit 120 fails, and a signal forsetting the first switching device 52 to the ON state is sent from therapid discharge control circuit 120 to the first switching device 52,even though a “command for setting the first switching device 52 to theOFF state” is sent from the integrated control device 100 (CPU) to therapid discharge control circuit 120.

Suppose that a short-circuit fault occurs. In this case, the voltageacross the opposite ends of the discharge current sensor (shuntresistor) 53 becomes larger than a threshold value for determiningshort-circuit fault. Accordingly, the abnormality detecting portion 130sends a signal indicative of this fact (occurrence of the short-circuitfault), to the integrated control device 100. As a result, the CPU makesan affirmative decision (“YES”) in step S220, and proceeds to step S230to send the above-described “resistor fusing high-voltage generationcommand signal” to the boost converter 30.

Namely, when the CPU proceeds to step S230, it sets the target valueVHtgt of the voltage VH between the output terminals of the boostconverter 30 (voltage between the pair of high-voltage-side terminalsP3, N3), to the “forcedly boosted voltage (e.g., 600V)”, irrespective ofa load condition of the load unit 12. Further, the CPU controls theboost converter 30 so that the voltage VH between the output terminalsof the boost converter 30 coincides with the target value VHtgt. As aresult, the voltage between the pair of high-voltage-side terminals P3,N3 is forcedly raised to the forcedly boosted voltage. This operation ofthe CPU will also be called “forced boosting operation”.

At this time, since the first switching device 52 remains in the “ON”state, current I (=VHtgt/RD) substantially flows through the dischargeresistor 51 where RD is a resistance value of the discharge resistor 51.It is to be noted that the resistance of the first switching device 52when it is in the “ON” state and the resistance of the discharge currentsensor 53 are sufficiently smaller than the value RD, and thus can beneglected.

In the meantime, the rating of the discharge resistor 51 is designed sothat the discharge resistor 51 fuses without fail if the “forcedboosting operation” lasts for a given period of time. As a result, thedischarge resistor 51 fuses, and the discharge current pathway of therapid discharge circuit 50 is cut off or disconnected, so that thevoltage between the pair of high-pressure-side terminals P3, N3 ismaintained. Accordingly, electric power can be kept supplied to the loadunit 12 (the first motor 81, second motor 82, etc.), thereby to keep thevehicle 10 running. Then, the CPU proceeds to step S295 to once finishthis routine.

After executing the forced boosting operation, the CPU continues tomonitor the result of determination from the abnormality detectingportion 130. When the result of determination is “a result indicatingfusing of the discharge resistor 51” (namely, when the voltage betweenthe opposite terminals of the discharge current sensor (shunt resistor)53 becomes smaller than the threshold value for determiningshort-circuit fault), the CPU may set the target value VHtgt of thevoltage VH between the output terminals of the boost converter 30 to “agiven value smaller than the forcedly boosted voltage”.

As explained above, the first power-supply unit 11 includes thehigh-voltage source HVS that generates a high voltage between thepositive electrode (terminal P3) and the negative electrode (terminalN3) so as to supply electric power to the load unit 12 connected to thepositive electrode and the negative electrode, the smoothing capacitor40 connected between the positive electrode and the negative electrode,the discharge portion DCHG (rapid discharge circuit 50) that isconnected between the positive electrode and the negative electrode andincludes the resistor (resistive element) 51 and the first switchingdevice 52 connected in series with each other, and the discharge controlportion (discharge control circuit) 120 that controls the firstswitching device 52 to any one of the “ON” state and the “OFF” state. Inthe first power-supply unit 11, when the first switching device 52 is inthe “ON” state, an electric charge stored in the smoothing capacitor 40is discharged by means of the discharging portion DCHG (rapid dischargecircuit 50). The first power-supply unit 11 further includes a forcedlycutting-off portion (the integrated control device 100, step S210-stepS230 of FIG. 2) that controls the high-voltage source HVS so that itcontinues to generate a given high voltage (forcedly boosted voltage) soas to fuse the resistor 51, when an abnormal condition in which electriccurrent (current equal to or larger than a value corresponding to thethreshold value for determining a short-circuit fault (abnormalcondition)) flows through the resistor (discharge resistor) 51 isdetected while the discharge control circuit 120 controls the firstswitching device 52 so as to keep the first switching device 52 in the“OFF” state.

Accordingly, when a short-circuit fault occurs to the rapid dischargecircuit 50 of the vehicle 10, a current that exceeds the rated currentof the discharge resistor 51 is caused to flow through the dischargeresistor 51 in the rapid discharge circuit 50, so as to fuse thedischarge resistor 51. Namely, the discharge current pathway is forcedlycut off, and discharging is stopped. In other words, the dischargeresistor 51 itself has the function of shifting the rapid dischargecircuit 50 from the short-circuited condition (abnormal condition) tothe forced cut-off condition. Accordingly, the power-supply unit 11 isable to forcedly cut off the discharge current pathway when an abnormalcondition is detected, without requiring a new component(s) to be addedto the rapid discharge circuit 50. Thus, even in the event of ashort-circuit fault, electric power can be supplied to the load unit 12,so as to enable the vehicle 10 to run.

A method of designing the resistance value RD and rating of thedischarge resistor 51 that can be fused without fail in the forcedlyboosting operation as described above will be described below.

Initially, a normal operation of the rapid discharge circuit 50 at thetime of a collision of the vehicle 10 will be considered. If the vehicle10 comes into collision, and the collision detecting portion 110operates normally, the integrated control device 100 generates a commandto place the relays SMR1-SMR3 in the “OFF” states. Then, the relaysSMR1-SMR3 are placed in the “OFF” states, and supply of input voltage tothe boost converter 30 is stopped. Further, the integrated controldevice 100 generates a command to place the first switching device 52 ofthe rapid discharge circuit 50 in the “ON” state. At this time, anelectric charge stored in the smoothing capacitor 40 is discharged. Thefollowing are conditions under which the rating of the dischargeresistor 51 is designed.

-   Design Conditions (normal time)

Maximum output value of the boost converter 30: VH=600V

Initial voltage value at the time of discharge: VH=600V  (1)

Target voltage value of discharge: VH=60V after 5 sec.  (2)

Under the above-indicated conditions (1), (2), the voltage V duringdischarge is expressed by the following equation, where t (sec.)indicates time (see FIG. 3).

V=600 exp(−0.46 τ)

-   Accordingly, the time constant T is determined as follows.

τ=1/0.461=2.17 (sec.)

The resistance value RD of the discharge resistor 51 showing the abovedischarge characteristics is determined as follows, where CS denotes thecapacitance value CS of the smoothing capacitor 40.

RD=τ/CS

Then, under the above-indicated conditions, the Joule-integral value I²tduring discharge is obtained (electric current during discharge isregarded as being proportional to the voltage VH between the terminals).A general formula for the Joule-integral value I²t is expressed by thefollowing equation, where i (t) indicates current.

I ² t=∫i ²(t)dt

In the case of charge/discharge waveform that makes an exponentialtransition, the Joule-integral value I²t₁ is expressed by the followingequation (3).

I ² t ₁=(½)·(VH/RD)²·τ  (3)

While the I²t value of the actual discharge resistor 51 is selectedbased on the value of the above equation (3) in view of the temperaturederating, or the like, the one of the minimum rating is normallyselected, in the light of the component cost and component size.

Then, suppose that the output voltage VH of the boost converter 30 isforcedly fixed to 600V. The discharge waveform in this case may beconsidered as a rectangular waveform. In the case of rectangularwaveform, the Joule-integral value is expressed by the followingequation (4), where t (sec.) indicates time.

I ² t ₂=(VH/RD)² ·t  (4)

The time t when the Joule-integral value of the above equation (4)coincides with the integral value of the above equation (3) is expressedas follows.

t=τ/2

Accordingly, where the voltage is a constant value of 600V, the resistorfuses when the time (t) starts being longer than τ/2. It is, however, tobe understood that the above-described derating is not taken intoconsideration, for the sake of simplicity.

If the discharge resistor 51 designed as described above is used in therapid discharge circuit 50, the Joule-integral value of the dischargeresistor 51 exceeds the rated value upon a lapse of about (τ/2) sec.after start of the forced boosting operation in which VH is fixed to600V, and the discharge resistor 51 fuses.

Second Embodiment

(Configuration) Next, a power-supply unit 11A (which will also be called“second power-supply unit”) according to a second embodiment of theinvention will be described. As shown in FIG. 4, the second power-supplyunit 11A is applied to the hybrid vehicle 10, like the firstpower-supply unit 11. In the following description, the same referencenumerals as used in the description of the first embodiment are assignedto the same or corresponding constituent elements or steps as those ofthe first embodiment.

The second power-supply unit 11A is different from the firstpower-supply unit 11, only in that a second switching device 54 isprovided in the discharge portion DCHG, and that, in the event of ashort-circuit fault, the second switching device 54 is switched from an“ON” state to an “OFF” state, instead of execution of the forcedlyboosting operation during a short-circuit fault. In the following, thesedifferences will be mainly described.

The second switching device 54 is connected in series with the dischargeresistor 51 and the first switching device 52. The second switchingdevice 54 is a power MOSFET, like the first switching device 52. Thesecond switching device 54 is adapted to change from the “ON” state tothe “OFF” state, based on a “cut-off command signal” from the integratedcontrol device 100.

When the integrated control device 100 receives a collision detectionsignal from the collision detecting portion 110, it sends a “dischargecommand signal” to the rapid discharge control circuit 120. Further, theintegrated control device 100 controls the second switching device 54 tothe “ON” state while the vehicle 10 is running. However, when theabove-described short-circuit fault occurs, the integrated controldevice 100 is configured to send the “cut-off command signal” to thesecond switching device 54, based on a signal from the abnormalitydetecting portion 130,

(Operation) Next, the operation of the second power-supply unit 11Aconstructed as described above will be described. At the time of acollision of the vehicle 10, the second power-supply unit 11A operatesin the same manner as the first power-supply unit 11 as described above.In the following, the case where a short-circuit fault occurs will bedescribed.

<Case of Short-circuit Fault> As described above, when the short-circuitfault as described above occurs, the integrated control device 100 sendsthe “cut-off command signal” to the second switching device 54, based onthe signal from the abnormality detecting portion 130. This point willbe described in more detail with reference to the flowchart of FIG. 5.

The CPU of the integrated control device 100 executes a routineillustrated in the flowchart of FIG. 5 each time a given length of timeelapses. Thus, at an appropriate time, the CPU starts the routine fromstep S500 of FIG. 5, and proceeds to step S210 to determine whether theCPU sends an “OFF” command to the first switching device 52 of the rapiddischarge circuit 50. In other words, the CPU determines whether “nodischarge command signal is generated” at this point in time.

If the vehicle 10 is in a collision as described above, the CPU sends acommand signal for placing the first switching device 52 in the ON stateto the rapid discharge control circuit 120. Namely, the CPU generates adischarge command signal. In this case, the CPU makes a negativedecision (“NO”) in step S210, and directly proceeds to step S595 to oncefinish the routine.

If the vehicle 10 is not in a collision but in a normal runningcondition, the CPU sends a signal for controlling the first switchingdevice 52 to the OFF state, to the rapid discharge control circuit 120.In this case, the CPU makes an affirmative decision (“YES”) in stepS210, and proceeds to step S220 to determine whether the result ofcomparison transmitted from the abnormality detecting portion 130indicates “occurrence of a short-circuit fault (abnormal condition)” asdescribed above.

Suppose that a short-circuit fault occurs. In this case, the abnormalitydetecting portion 130 sends a signal indicative of this fact (occurrenceof the short-circuit fault) to the integrated control device 100. As aresult, the CPU makes an affirmative decision (“YES”) in step S220, andproceeds to step S510.

If the CPU proceeds to step S510, it sends the above-described “cut-offcommand signal” to the second switching device 54. As a result, thesecond switching device 54 switches from the “ON” state to the “OFF”state. Namely, the discharge current pathway of the rapid dischargecircuit 50 is cut off, so that the voltage between the pair ofhigh-voltage-side terminals P3, N3 is maintained. Accordingly, electricpower can be kept supplied to the load unit 12 (the first motor 81, thesecond motor 82, etc.), so as to keep the vehicle 10 running.Thereafter, the CPU proceeds to step S595, to once finish the routine ofFIG. 5.

As explained above, the second power-supply unit 11A includes thehigh-voltage source HVS that generates a high voltage between thepositive electrode (terminal P3) and the negative electrode (terminalN3) so as to supply electric power to the load unit 12 connected to thepositive electrode and the negative electrode, the smoothing capacitor40 connected between the positive electrode and the negative electrode,the discharge portion DCHG (rapid discharge circuit 50) that isconnected between the positive electrode and the negative electrode andincludes the resistor (resistive element) 51 and the first switchingdevice 52 connected in series with each other, and the discharge controlportion (discharge control circuit) 120 that controls the firstswitching device 52 to any one of the “ON” state and the “OFF” state. Inthe power-supply unit 11A, when the first switching device 52 is in the“ON” state, an electric charge stored in the smoothing capacitor 40 isdischarged by means of the rapid discharge circuit 50. Further, therapid discharge circuit 50 includes the second switching device 54connected in series with the resistor (discharge resistor) 51 and thefirst switching device 52. The power-supply unit 11A further includes aforcedly cutting-off portion (the integrated control device 100, stepS210—step S510 of FIG. 5) that switches the second switching device 54from the “ON” state to the “OFF” state, when an abnormal condition (anabnormal condition in which the first switching device 52 is placed inthe “ON” state) in which electric current (current equal to or largerthan a value corresponding to the threshold value for determining ashort-circuit fault (abnormal condition)) flows through the resistor 51is detected while the discharge control circuit 120 controls the firstswitching device 52 so as to keep the first switching device 52 in the“OFF” state.

Accordingly, when a short-circuit fault occurs to the rapid dischargecircuit 50 of the vehicle 10, the second switching device 54 in therapid discharge circuit 50 switches from the “ON” state to the “OFF”state, so that the discharge current pathway is forcedly cut off, anddischarging is stopped. Accordingly, the second power-supply unit 11A isable to cut off the discharge current pathway instantly (within aresponse time of the second switching device 54), so as to accomplishthe intended object.

The invention is not limited to the above-described embodiments, butvarious modified examples may be employed within the scope of theinvention. For example, the vehicle 10 may be an electric vehicle. Also,the boost converter 30 may be a voltage converting device of a typeother than that as illustrated above. In addition, the collisiondetecting portion 110 may be a known ECU and sensor for control of anair-bag system. Also, the rapid discharge control circuit 120 maydirectly receive a collision detection signal from the collisiondetecting portion 110, and switch the first switching device 52 from the“OFF” state to the “ON” state.

What is claimed is:
 1. A power-supply unit comprising: a high-voltagesource configured to generate a high voltage between a positiveelectrode and a negative electrode so as to supply electric power to aload unit connected to the positive electrode and the negativeelectrode; a smoothing capacitor connected to the positive electrode andthe negative electrode; a discharge portion that includes a resistor anda first switching device connected in series with each other, and isconnected to the positive electrode and the negative electrode; adischarge control portion configured to control the first switchingdevice to one of an ON state and an OFF state; an abnormality detectingportion configured to detect occurrence of an abnormal condition inwhich electric current flows through the resistor even though thedischarge control portion controls the first switching device so as tokeep the first switching device in the OFF state; and a forcedlycutting-off portion configured to forcedly cut off a discharge currentpathway formed by the discharge portion, when the abnormal condition isdetected, wherein the smoothing capacitor and the discharge portion areconfigured such that an electric charge of the smoothing capacitor isdischarged by the discharge portion when the first switching device isin the ON state.
 2. The power-supply unit according to claim 1, whereinwhen the abnormal condition is detected, the forcedly cutting-offportion is configured to control the high-voltage source so that thehigh-voltage source keeps generating a given high voltage for fusing theresistor of the discharge portion.
 3. The power-supply unit according toclaim 1, wherein: the discharge portion includes a second switchingdevice connected in series with the resistor and the first switchingdevice; and the forcedly cutting-off portion is configured to switch thesecond switching device from an ON state to an OFF state when theabnormal condition is detected.